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Learning Objectives Describe the path of nerve impulses in reflex arcs Describe the path of nerve impulses in reflex arcs Describe the pupil reflex Describe the pupil reflex Explain the process by which the pupil reflex is controlled Explain the process by which the pupil reflex is controlled Key Notes Interesting FactsQuestions

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Reflex Actions Many of the actions we carry out every day are done without conscious thought Many of the actions we carry out every day are done without conscious thought These are call Reflex Actions These are call Reflex Actions The nerve impulses in reflex actions do not follow the same path as those in conscious actions The nerve impulses in reflex actions do not follow the same path as those in conscious actions The path they follow is called the Reflex Arc The path they follow is called the Reflex Arc

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Interlink/Relay Neurons These neurons are the vital part of the reflex arc These neurons are the vital part of the reflex arc They connect the Sensory neuron to the Motor/Effector neuron They connect the Sensory neuron to the Motor/Effector neuron They can be found in the grey matter of the spinal cord and in the midbrain They can be found in the grey matter of the spinal cord and in the midbrain

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Advantage of Reflexes The reflex arc allows a much quicker response to stimuli The reflex arc allows a much quicker response to stimuli This is especially important where the stimuli is a source of potential damage e.g. a hot object, a sharp pin This is especially important where the stimuli is a source of potential damage e.g. a hot object, a sharp pin

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Stimulation of the Reflex Response The speed of the reflex response can be increase by several factors: The speed of the reflex response can be increase by several factors: Exposure to adrenaline (Sympathetic Nervous System) Exposure to adrenaline (Sympathetic Nervous System) Exposure to stimulant drugs (Caffeine, Beta Amphetamines/Speed) Exposure to stimulant drugs (Caffeine, Beta Amphetamines/Speed)

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The Pupil Reflex Response The pupil response allows the iris to contract and dilate The pupil response allows the iris to contract and dilate This changes the size of the pupil This changes the size of the pupil In the eye the pupil acts like an aperture In the eye the pupil acts like an aperture It controls the amount of light which enters the eye.. It controls the amount of light which enters the eye.. … and protect the retina from “overexposure” … and protect the retina from “overexposure”

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Bright Light, Bright Light When light levels are high Circular muscles in the iris contract When light levels are high Circular muscles in the iris contract This causes the pupil to become smaller This causes the pupil to become smaller This is controlled by the parasympathetic nervous system This is controlled by the parasympathetic nervous system

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In the Dark… When light levels are low Radial muscles contract When light levels are low Radial muscles contract This causes the pupil to increase in size This causes the pupil to increase in size This is caused by the sympathetic nervous system This is caused by the sympathetic nervous system

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Diagram of Pupil Response

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Control of the Pupil Reflex: In Detail When high levels of light strike photoreceptors in the retina they cause nervous impulses to be sent along the Sensory neuron (Optic Nerve) When high levels of light strike photoreceptors in the retina they cause nervous impulses to be sent along the Sensory neuron (Optic Nerve) These impulses are co-ordinated by relay neurons in the Midbrain These impulses are co-ordinated by relay neurons in the Midbrain Signals are then sent along the parasympathetic nervous system Signals are then sent along the parasympathetic nervous system

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Atropine Atropine is a sedative drug made from the deadly nightshade plant (Atropa belladonna) Atropine is a sedative drug made from the deadly nightshade plant (Atropa belladonna) It was apparently used in the middles ages by women to dilate there pupils It was apparently used in the middles ages by women to dilate there pupils This is the source of the word belladonna in Latin and Belle in French This is the source of the word belladonna in Latin and Belle in French